Reprobate’s drunk treble in one (9)
I believe the answer is:
libertine
'reprobate's' is the definition.
(I've seen this before)
'drunk treble in one' is the wordplay.
'drunk' indicates anagramming the letters.
'one' becomes 'i' (roman numeral).
'treble'+'in'+'i'='trebleini'
'trebleini' anagrammed gives 'LIBERTINE'.
(Other definitions for libertine that I've seen before include "Rakish" , "Rake" , "Rou" , "Debauchee" , "Sensually indulgent person" .)